The present invention relates to the cueing, or positioning, of a location on a video tape moved by a tape transport. More particularly, the present invention relates to positioning a cue location on the video tape so that the cue location arrives at its destination with the tape moving at a selectable velocity.
In video production, video tape recorders are used to store video information. Video information is composed of frames of video images. Each frame is in turn composed of two fields of video information. Video information is stored in discrete locations on the video tape. In the commonly used Type C video tape format, each field of video information is stored on the video tape in a helical track. In addition to these tracks of video information, there are other signals recorded on the video tape. These include control track, time code, and audio signals. Control track is typically a periodic signal on the tape used to accurately locate the beginning of each frame of video information.
Video tape recorders perform a number of operations including the playing and recording of video information. To play or record video information, the tape is moved past a scanner, which contains transducers such as a play head, a record head, and an erase head. While the tape moves past the transducers, video information is either read with the play head, written with the record head, or erased with the erase head. The video information is sent to or received from a signal system of the video tape recorder, which interfaces between the video information stored on the video tape and the outside world.
Video tape recorders are used to play and record video information that is stored at specific locations on the video tape. For example, it is common to record a large amount of video information during a video production. Only a small portion of the information is usually needed. The cue function is used to locate the beginning of the portion of needed video information stored on the tape. Similarly, it is often necessary when recording video information to begin recording at a specific location on the tape, for example, following a portion of video information previously recorded. The cue function of a video tape recorder is used to do this positioning of the video tape.
The purpose of the cue function on a video tape recorder is to move a specific location on the video tape to the scanner, so that it can be played, recorded, or erased. This function is also known as the search function. For the purposes of the disclosure, this function will be called the cue function. This location is called the cue location, and the moving of the cue location to the scanner is called cueing. The cue location marks the beginning of a portion of video information to be played, recorded, or erased. This portion of video information might be as short as a field of video information or as long as many thousands of video frames.
The cue function is implemented in present video tape recorders as follows. It is determined whether the cue location lies ahead of or behind the location on the tape presently at the scanner. If it is ahead, the tape transport accelerates the tape in the forward direction. If it is behind the present location, the tape transport accelerates the tape in the reverse direction. As the cue location approaches the scanner, the transports decelerates the tape. Ideally when the cue location reaches the scanner, the tape has been decelerated to a velocity of zero. Less accurate video tape recorders tend to miss the cue location on the first try and have to hop around the location until it is hit.
Actually, arriving at the cue location at zero velocity can waste time because what is often desired is that the tape be moving at the velocity at which the playing or recording is to take place. A disadvantage of the present cue functions is that they can only cue to a zero velocity. Additionally, the transport cannot instantaneously jump the tape from zero to a desired velocity. Thus, the solution in present day recorders is to cue to a location a specific number of frames behind the actual cue location. From that location, the tape is accelerated from a zero velocity to the desired velocity in the distance before the actual cue location is reached. In effect, the transport takes a running start at the actual cue location. There are actually a number of other reasons to choose a location ahead of the actual cue location including allowing time for the various control systems to lock in place, and allowing the operator a visual reference before the cue location.
There is another type of cueing operation that cannot be conveniently executed by present video tape recorders. This function is called synchronous cueing. Synchronous cueing differs from standard cueing in that the cue location changes during the cue operation. In synchronous cueing, the tape is cued to a moving target as the cue location is advanced at the rate of the desired velocity. The advancing cue location simulates a moving reference such as another video tape recorder. Tape movement of the tape recorder is synchronized with this simulated moving reference.
Synchronous cueing can be used to synchronize the playing of video information by one video tape recorder with other video tape recorders or other devices. An example of this would be in a presentation using two or more video tape recorders which must be synchronized. To perform this synchronization with present video tape recorders, the present cue function would be used. After each recorder has been separately cued to the cue location for each recorder, all the recorders would be cued up to play speed. Using this method, time must be allowed for each recorder to cue, before the recorders will be synchronized. While this may be necessary in specific instances, this method is complex and requires that all the tape recorders wait for the last recorder to cue, which potentially may waste a great deal of time.
Therefore, a need exists for a cueing technique in which a cue location can arrive at the scanner, at a desired velocity. There is also a need for a synchronous cueing technique in which the cue location changes during the cue operation at the rate of the desired velocity.